1. Field of Invention
This invention is in the field of optoelectrical characterization of integrated circuits (IC) using scanning or/and stepping (touching) nanoprobing systems.
2. Related Art
Nanoprobing covers a broad field of analytical science including various types of dimensional, electrical, mechanical, compositional and chemical physical characterization of nanoobjects. Nanoelectronic devices, such as advanced (<130 nm) ICs, are examples of such objects.
Conventional far-field optical probing, which uses 1000-1500 nm laser, is running out of resolution. Currently even the most sophisticated photon delivery and collection optics (solid immersion lens SIL) provides lateral resolution of about 200 nm and with some even greater efforts about 100 nm. Required spatial resolution of optics is about twice of the minimum gate (or contact level) pitch. The pitch dimension of transistors at technology nodes is about 140 nm for the 20 nm node, 100 nm for the 14 nm node, 70 nm for the 10 nm node and 50 nm for the 7 nm node; these dimensions are also the requirements for the resolution of optical probing of transistors. Therefore, resolution of optical commercial/industrial probers must be improved to follow Moore's Law—the industry trend.
Near-field scanning optical microscopy (NSOM) is a known way of improving resolution of optics beyond the diffraction limit. This solution has a serious limitation related to strong dependence of photon collection efficiency on the ratio of aperture diameter to wavelength (a fourth power dependence by Bethe's theory). For 1250 nm unpolarized photons (the middle of 1000 nm to 1500 nm range currently used by optical circuit analysis) and 200 nm aperture diameter, efficiency is close to 1.5%, for 50 nm aperture it is only 0.006%. At most only one of 17,000 photons emitted by sample is collected. For practical applications photon collection efficiency is inadequate.
To overcome diffraction limits of optical microscopy resolution, various near-field evanescence radiation approaches were used in the past. Example: NSOM which uses fiber opening with dimensions less than the diffraction limit. In the near-field case, resolution of the system is defined by the aperture of optical probe (fiber, pin hole, etc.). The problem with any near-field evanescence method is its poor photon delivery and collection efficiency. Collection efficiency of NSOM with fiber probes is falling with diameter of the fiber aperture or with its spatial resolution as D3 (experiment) or even D4 (theory) function. For 1250 nm light expected transmission of 100 nm pin hole is about 0.0001 and for 50 nm resolution one should expect 0.00006 transmissions. This even further reduces the method's throughput and makes fiber NSOM-based high-resolution optical circuit analysis (OCA) simply impractical.
The goal is to collect every possible photon interacted with or emitted by the targeted transistor/diode and yet to preserve required spatial resolution. The near-field transducers (NFT) or/and optical nano-antennas have been used to concentrate optical energy in spot size less than the diffraction limit. This recent NFT development is supported by data storage companies because the heat assisted magnetic recording (HAMR) technology promises to achieve higher densities of data storage. Resolution of ˜20 nm in near-field can be achieved today using various NFT's with transmission at 800 nm wavelength (or coupling with magnetic media efficiency) from a few to tens of percent. Note the wavelengths needed with NSOM for optical probing can be shorter than for far field probing; however, far field probing can work with silicon thicknesses of more than 10 um, whereas NSOM must work with silicon thicknesses of less than 250 nm. These numbers should be compared with transmission of simple metal aperture of 20 nm in diameter which is about 0.0002%. Therefore, NFT's significantly improve efficiency (transmission, coupling efficiency) of near-field optics.
The spatial resolution of an imaging far-field optical system used to collect photons from multiple points of the region of interest (ROI) with a laser scanning system is limited fundamentally by what is called the diffraction limit, defined by Ernest Abbe. This spatial resolution depends on wavelength, numerical aperture as well as quality of optical system and emission, reflection or absorption properties of the sample. This same diffraction limit restricts the reduction of a laser probe below a certain size. This limit is again defined by wavelength, numerical aperture and quality of focusing optics. A few techniques are known which help to overcome the diffraction limit to resolution of an optical system. One of them is scanning or positioning a nanoscale photon sensor/source in the near-field of the ROI. NSOM in which an aperture of conductor coated optical fiber defines the “sensor/source” size can be used. Despite poor transmission of a thin fiber, this type of NSOM is sometimes employed to deliver photons to a ROI with a nanoscale resolution (high power of a source laser helps). However, use of NSOM for collection of emitted or reflected photons is limited. Theoretical photon collection efficiency of sub-wavelength aperture drops as the fourth power of diameter to wavelength ratio. Some experimental data suggests slightly less abrupt decay of etched and metal coated fiber transmission—as the third power of diameter to wavelength ratio. Even in this third power case going from 250 nm resolution provided by far-field optics to 50 nm resolution of NSOM will cause signal reduction of more than 2 orders of magnitude (1/125 or 0.008). Considering sequential data collection algorithms of NSOM one faces a significant loss of throughput going from parallel imaging with 250 nm resolution to sequential scanning microscopy with 50 nm resolution (only extra data collection time can improve the signal-to-noise ratio, SNR). Photon collection efficiency (or transmission) of the nanoscale, sub-wavelength optics must be significantly improved for the method to be accepted for industrial applications.
Accordingly, there is a need in the art to enable probing of IC's at the upcoming design nodes, which cannot be probed using current technology due to insufficient resolution and/or photon collection efficiency. This disclosure describes the system and method for doing that.